UC IRVINE (US)—Burn tons of fossil fuels. Pump those chemicals into the air where they react with hydrochloric acid from airborne sea salt on buildings and roads. What you get is smog-forming chlorine atoms—and a recipe for choking air pollution—a new study finds.

Under extreme circumstances, this previously unknown chemistry could account for up to 40 parts per billion of ozone—nearly half of California’s legal limit on outdoor air pollution. This reaction is not included in computer models used to predict air pollution levels and the effectiveness of ozone control strategies that can cost billions of dollars.

“Realistically, this phenomenon probably accounts for much less than 40 parts per billion, but our results show it could be significant,” says Barbara Finlayson-Pitts, chemistry professor at the University of California-Irvine and lead author of the study.

“We should be monitoring it and incorporating it into atmospheric models,” she continues. “We still don’t really understand important elements of the atmosphere’s chemistry.”

Previously, scientists believed that nitrogen oxides generated when fossil fuels burned would be eliminated from the atmosphere when they came in contact with surfaces. Hydrochloric acid also is found indoors in cleaning products. When it interacts with nitrogen oxides from appliances such as gas stoves, chlorine compounds form that cause unusual chemistry and contribute to corrosion indoors.

Ozone is known to cause coughing, throat irritation, chest pain, and shortness of breath. Exposure to it has been linked to asthma, bronchitis, cardiopulmonary problems, and premature death.